The goal of this work is to use human RNases and homologous RNases from other species instead of toxic plant and bacterial proteins in the construction of immunotoxins. Two of the major problems with the clinical use of immunotoxins is the toxicity and immunogenicity of the toxins. The use of human RNases that acquire toxicity by targeting addresses these problems. Chemical conjugates of antibodies to the human transferrin receptor conjugated to bovine RNase A inhibited the growth of human glioma cells in an animal model as well as a ricin-A chain conjugate constructed with the same antibody. The IC50 of a recombinant chimeric mouse/human antibody to the human transferrin receptor fused to the gene for human angiogenin RNase to kill human leukemia cells was 5 X 10(-11) M. This compared very well with the in vitro potency of classical immunotoxins. A single chain antibody constructed from the chimeric anti-transferrin receptor antibody was fused to the gene for human eosinophil RNase and expressed in bacteria. This antibody enzyme construct specifically bound to its target cells, expressed specific RNase activity and killed human tumor cells with an IC50 of 5 X 10(-10) M. A homologous RNase (Onconase) from frog oocytes has inherent anti-tumor properties and is in clinical trials as an anti-cancer agent. Onconase has been administered to humans on a weekly basis for up to six months without causing immunological problems or serious toxicities, presumably because of its homology to human plasma RNases. The specificity of Onconase as an anticancer agent can be improved by targeting and to this end the gene has been cloned from Rana pipiens genomic DNA. Onconase has been demonstrated to have specific activity against HIV-1 and these results were confirmed in the NCI AIDS screen. Thus, the potent targeted cytotoxicity of these new reagents can also be directed to MDS therapies.